The successful prevention of homelessness demands an understanding of the broad context within which homelessness and interventions to combat it occur. The purpose of this study is to further our understanding of this context, and specifically to elucidate how homelessness in United States cities emerged among severely mentally ill (SMI) populations. The study will use historical methods to describe and compare the evolution of mental health and housing policies in New York and Los Angeles, and in the context of national policy development. We will consult primary written and oral sources to document the policies that have influenced urban homelessness in different locales, especially with regard to people with SMI, as well as the issues and decisions involved in local policy responses. A central concern will be the relative prominence of housing versus mental health services in local policy responses to homelessness among people with SMI. By illuminating the context within which key policy and program planning decisions are made, findings from this study have the potential to improve the likelihood that relevant, effective interventions will actually reach those in need. The study builds upon a pilot study conducted in 2006 under the auspices of the NIH-funded Columbia Center for Homelessness Prevention Studies. In this study the researchers used snowball sampling techniques to conduct unstructured interviews with policymakers, mental health professionals, and community leaders, in New York and other cities, who shaped responses to homelessness and mental illness between the 1960s and 1990s. Several preliminary findings have emerged from these interviews: a) that homelessness among the mentally ill was not perceived to result directly from deinstitutionalization;b) that homelessness was not named and recognized by policymakers until the late 1970s;and c) that diversity in policy toward SMI homeless populations in different cities resulted from the diverse legal, political, and geographical environments in which the policies were formed. The study will examine these initial findings in greater depth and will further trace how specific conditions and mental illness treatment approaches interacted with politico-social environments to produce the current landscape of homelessness, mental illness policy, and housing interventions.